1. Field of the Invention
The present invention relates to an air bag folded and contained in an air-bag system which is mounted on a vehicle and is used for passenger protection. In particular, the present invention relates to an air bag system which can develope the air bag not in a straight thin shape toward a passenger but in a round shape immediately after an inflator supplies gas into the air bag.
2. Description of the Related Art
Conventionally, an air bag system, as shown in FIG. 21, for protecting a passenger is known (Japanese Patent Application Laid-Open No. H.8-268198). In this air bag system 200, a base plate 21 made of metal is attached to a boss plate of a steering wheel (not shown) via a bracket or the like. An inflator attachment hole (not shown) is formed in the base plate 21. An inflator (not shown) is fit in the inflator attachment hole from a lower side of the base plate 21 in FIG. 21. On an upper side face of the base plate 21, a folded air bag 24 is attached. The folded air bag 24 is covered by a lapping sheet member 25, whereby the shape of the folded air bag 24 is maintained. A plurality of holes 25d is formed at each end of the lapping sheet member 25. The lapping sheet member 25 is fixed to the base plate 21 by inserting rivets into the holes 25d and holes formed in the base plate 21. A tear line 25c is formed at a substantially center part of the lapping sheet member 25.
Since the shape of the folded air bag 24 is maintained, when the folded air bag 24 is contained in a pad of the steering wheel, the pad of the steering wheel is prevented from expanding due to a restoring force of the folded air bag 24. Furhter, when the air bag 24 is developed, the lapping sheet member 25 is fractured at the tear line 25c due to pressure applied from the developing air bag 24.
However, according to the conventional air bag system as described above, a passenger may be strongly hit by the developing air bag 24. FIGS. 22A to 22D are schematic views illustrating a developing state of the air bag 24 contained in the pad 20 in a stepwise manner. The air bag 24 is first folded in a straight thin shape and then the both sides 24c of the straight thin shape are further folded plural times in perpendicular to a longitudinal direction of the straight thin shape. The air bag 24 thus folded is covered by the lapping sheet member 25 and is contained in the pad 20 (FIG. 22A).
When the inflator 80 is ignited and a gas produced by the inflator 80 flows into the air bag 24 upon detection of a vehicle collision, a part 24b of the air bag 24 in the vicinity of the inflator 80 expands due to pressure of the gas. As a result, the air bag 24 presses the pad 20 from the inside thereof (FIG. 22B). When the pressure applied from the air bag 24 to the pad 20 reaches a predetermined value, the pad 20 and the lapping sheet member 25 are fractured at tear lines 20a and 25c, respectively (FIG. 22C). As a result, a force for maintaining the shape of the folded air bag 24 is released at one time. For this reason, the air bag 24 is firstly developed in such a manner that side portions 24c of the air bag 24 folded in the straight thin shape rapidly project toward the passenger by energy of the gas stored in the part 24b of the air bag 24 (FIG. 22D), and then the air bag 24 is developed in a round shape.
As described above, according to the conventional air bag system, the side portions 24c of the air bag 24 vigorously project toward the passenger, that is, the air bag 24 is developed in a long thin shape toward the passenger. Such a developing form of the air bag 24 is not desirable because the developing air bag may injure the passenger.